An ad-hoc network is a network autonomously configured with communicable nodes without any fixed base station (BS) or access point (AP). In the ad-hoc network, since most nodes support mobility, each node does not receive a continuous energy supply but uses a battery with a limited capacity as an energy source. Particularly, among several main factors that nodes use energy, energy consumed in communication has a great deal of weight.
As a representative method for reducing energy consumed in communication, there is a method in which when communication between nodes is necessary, the nodes wake up in a communicable state, and on the contrary, when nodes are in a standby state in which communication between the nodes is unnecessary, the nodes enter into a sleep state, thereby reducing energy consumption.
However, in the method, it is very important that all nodes existing in a network simultaneously wake up and enter into the sleep state while having the same time information, and a technique for synchronizing time between nodes is required.
A time synchronization function (TSF) algorithm described in the 802.11 standards is used as a representative method of time synchronization protocols, which is proposed in a conventional art. In the TSF algorithm, when nodes exchange time information with each other by transmitting/receiving beacon signals between the nodes, if a beacon signal received by a node has time information earlier than time information of the node, synchronization is performed while correcting the time information of the node to the early time information. If a beacon signal received by a node has time information later than time information of the node, synchronization is not performed.
In the conventional art, its procedure is simple, and thus time synchronization is well performed when a small number of nodes exist in a network. However, in a network having a large number of nodes, since the number of nodes competing in a beacon contention window increases, the opportunity that a node having early time information can transmit/receive its own beacon signals is reduced. Hence, there is a problem in performing time synchronization of the entire network.